This invention claims priority of a German patent application DE 100 11 202.1 which is incorporated by reference herein.
The invention refers to the field of electron beam lithography, in particular to a method for directing the electron beam onto a target position on the surface of a substrate, the substrate first being placed onto a movable stage (2) and the stage then being displaced stepwise, in the X and/or Y coordinates of a Cartesian grid, into predefined positions.
Methods of this kind are used in order to write linear or planar patterns onto the radiation-sensitive resist layer of substrates, which can then be used, for example, as phase masks for chip manufacture.
In conventional methods, the electron beam is directed onto the target position either by converting location information about the target point into a deflection of the electron beam, or by shifting a stage that carries the substrate and is movable in stepwise fashion.
An apparatus for electron beam lithography in which the substrate is positioned by means of a stage with respect to an approximately stationary electron beam is known, for example, from Microelectronic Engineering 27 (1995) 135-138. This apparatus allows a positioning accuracy for the stage, and thus for the target position relative to the electron beam, of approximately 10 nm to 2.5 nM. A further increase in the positioning accuracy of the stage is possible only with considerable outlay in terms of precision equipment.
In practice, however, in specific cases the positioning accuracies that are required lie beyond the accuracies attainable, even with the greatest effort, using a stage movable in stepwise fashion. One example of such a case is the production of phase masks for chirped Bragg gratings (or fiber Bragg gratings), which require positioning accuracies of at least 0.1 nm to 0.05 nm; this accuracy must moreover be available over a displacement range of approximately 150 mm.
In a grating of this kind having a grating period of 1 xcexcm and a width of 100 mm, 100,000 parallel grating lines must be applied in a highly accurate arrangement with respect to one another. The special feature of chirped gratings is that the spacing between the last two grating lines must be, for example, 0.5 run greater than the spacing between the first two grating lines. In addition, the spacing between adjacent grating lines must increase linearly from the first two grating lines to the last two grating lines. Theoretically, this means that the spacing between two adjacent grating line pairs must increase by 0.005 pm in each case. This difference is on the order of a fraction of an atomic diameter, and thus cannot be realized with physical equipment.
Given the large number of grating lines, however, the exact position of the grating lines is not important, since the grating acts collectively. Instead, the real position of the grating lines can fluctuate statistically about the theoretical reference position with no immediate risk of thereby losing the overall functionality of the grating. Assuming a Gaussian distribution for the fluctuation, even 1"sgr" values of less than 2 nm would be entirely permissible.
The functionality of the chirped grating is lost, however, if systematic deviations in the position of the grating lines occur, or if many grating lines are offset by the same magnitude.
Theoretical calculations have indicated that sufficient functionality in a chirped grating can just be achieved if the grating is broken down into line packets of no more than 200 grating lines each, and if the average grating constant from one packet to another then decreases by the value that would result, under the above circumstances, for 200 lines, namely by 1 pm (10xe2x88x9212 m) in this case. This change can be effected if, in each successive packet, the spacing of only one line pair is decreased by one step or one increment of a positioning system used to produce it. The step distance or increment of the positioning system would then need to be approximately 0.2 nm. This is not achievable, however, with previously known apparatuses for electron beam lithography.
To produce a grating having the aforementioned functionality, it is also known to divide it first into a plurality of working fields arranged one behind another, each of which has a predefined number of grating lines, for example 500 grating lines for a field size of 500 xcexcm. The working fields are then scaled in size, the difference in size between two adjacent working fields being approximately 1.25 nm. In a grating of this kind, all the lines in one working field theoretically have the same spacing. At the transition to an adjacent working field, i.e. every 500 lines, the line spacing theoretically jumps by a value of 2.5 pm.
The principal problem here is accurate assembly of the working fields one behind another, since the assembly accuracy of conventional apparatuses for electron beam lithography lies in the nanometer range. Assembly errors between the working fields become perceptible, however, as systematic errors due to higher-order structures in the grating. Additional systematic deformations of the grating result from residual distortions of the working field, so that the deformations repeat with the periodicity of the working fields.
Proceeding therefrom, it is the object of the invention further to improve positioning accuracy in electron beam lithography.
This object is achieved with a method for directing an electron beam onto a target position on the surface of a substrate in which firstly the substrate is placed onto a stage that is movable in stepwise fashion in the X and Y coordinates in a Cartesian grid, then the stage is displaced until the target position is located at a spacing from the impact point of the undeflected electron beam which is smaller than the smallest step distance of the stage displacement system, and then the electron beam is directed onto the target position by deflection.
By distributing the position adjustment between two positioning system, i.e. positioning firstly with the stage displacement system and secondly with the beam deflection system, the method according to the present invention makes it possible to achieve accuracies of up to 0.05 nm. With it, a grating having the functionality of the chirped Bragg grating described above can be configured on a substrate without difficulty and with high quality.
The method according to the present invention is not limited to the manufacture of this kind of grating. Rather it is suitable for all applications in which extreme precision in establishing the target point on the substrate surface is important. A preferred field of application is the writing of linear or planar patterns for phase masks. The method can also be used for direct exposure of such patterns on semiconductor substrates.
Distribution of the positioning task between two serially arranged positioning systems furthermore allows the use of a conventionally operated substrate stage for coarse adjustment of the target position, which is physically separate from the fine adjustment by means of deflection of the electron beam. Despite the increase in positioning accuracy, the equipment outlay can thus be kept relatively low.
The step distance of the movable stage preferably lies in the range from 1 nm to 10 nm and in particularly preferred fashion is 2.5 nm, i.e. it lies within the accuracy range of conventional apparatuses for electron beam lithography, which thus can easily be expanded, by appropriate configuration of the system controller, to include the method according to the present invention.
In an advantageous embodiment, the deflection of the electron beam is scaled to a range of xc2x13 xcexcm to xc2x16 xcexcm. The deflection range of the electron beam is thus confined to a very narrow subregion on the substrate in which, with a moderate hardware outlay, a very high addressing and working accuracy for precision positioning is then available. The positioning accuracy is achieved in both coordinates (X and Y) of the motion plane of the stage, so that even non-straight-line patterns can be written very accurately. For very high-accuracy addressing of the target position, it is also conceivable to scale the deflection range to the step distance of one individual increment of the stage.
For writing parallel grating lines, in an embodiment that is particularly advantageous for that purpose the stage is moved stepwise in the X and Y coordinates of a plane. In this context, the deflection of the electron beam is scaled in a range of xc2x13 xcexcm to xc2x16 xcexcm in only one of the X or Y coordinates. This allows the particularly sensitive spacing of the grating lines from one another to be established with high accuracy. On the other hand, the outlay remains low for the less critical setting of changes in the target position in the longitudinal direction of the grating lines once the line spacing (i.e. the coordinate transverse to the grating lines) has been set. The time required to write the grating can also be kept short. Lines are written in their longitudinal direction simply by shifting the stage in the relevant coordinate X or Y.
In an alternative embodiment, on the other hand, deflection of the electron beam is also implemented in the longitudinal direction of the grating lines and is then, in the interest of efficient hardware utilization and a rapid writing rate, preferably scaled in that coordinate to a larger range of xc2x118 xcexcm to xc2x1180 xcexcm.
In a further advantageous embodiment of the invention, the deflection of the electron beam is performed with a resolution of 16 bits. This allows a rapid switchover to be effected between differently scaled deflection ranges of the electron beam, a finer resolution in path magnitudes being achieved as the deflection ranges become smaller. As a consequence, even very accurately calculated target positions can be converted into a correspondingly precise deflection signal for the electron beam.